The present invention relates to liquid crystal display devices. More specifically, it relates to liquid crystal display devices each having a back light as a light source, which light source includes a green-emitting phosphor that emits light of high brightness with less variation with temperature.
Liquid crystal display devices generally include a back light unit 1 and a liquid crystal display panel 2, as illustrated in an exploded perspective view in FIG. 8. The back light unit 1 includes a light source 5, drive circuits (inverters) 9 configured to drive the light source 5, a casing 3, a reflector 4, a diffuser plate 6, a prism sheet 7, and a reflective polarizer 8.
The liquid crystal display devices produce color images in the following manner. The back light unit 1 is so configured to guide light emitted from the light source 5 toward the liquid crystal display panel 2. The liquid crystal display panel 2 is so configured to control the transmittance of the guided light on a pixel basis and to split the light into red, green, and blue components and transmit one of red, green, and blue light on a pixel basis.
Cold cathode fluorescent lamps (CCFLs) are generally used as light sources of liquid crystal display devices. FIG. 9 is a sectional view of a cold cathode fluorescent lamp in a longitudinal direction. The cold cathode fluorescent lamp includes a glass tube 11, a phosphor 12 arranged on an inner wall of the glass tube 11, and electrodes 13 arranged at both ends of the glass tube 11. In addition, a discharge medium 14 is enclosed in the glass tube 11. The discharge medium 14 contains, for example, mercury (Hg) and a rare gas such as argon (Ar) or neon (Ne).
A very thin and long shape is a feature of such cold cathode fluorescent lamps generally used as back lights (i.e., light sources) of liquid crystal display devices, which shape differs from those of fluorescent lamps for indoor lighting. Fluorescent lamps for indoor lighting generally have a diameter (inner diameter) of tube of about 30 mm and a length of about 1100 mm. Such a diameter (inner diameter) of tube is simply referred to as “diameter (inner diameter)”. In contrast, cold cathode fluorescent lamps generally have a very small diameter (inner diameter) of about 4 mm and a length of about 720 mm in the case of those for 32-inch liquid crystal display devices. Such a very small diameter is a feature of cold cathode fluorescent lamps.
The cold cathode fluorescent lamp illuminates upon the application of a high voltage to the electrodes 13 at both ends. The applied voltage causes electron emission from the electrodes, the emitted electrons excite mercury (Hg), and the excited mercury (Hg) emits ultraviolet rays when it returns to a ground state. The phosphor is excited by the action of the ultraviolet rays to thereby radiate visible rays toward the outside of the cold cathode fluorescent lamp.
The phosphor 12 in the cold cathode fluorescent lamp includes a mixture of powders of a blue-emitting phosphor, a green-emitting phosphor, and a red-emitting phosphor in such proportions to emit light of a predetermined white chromaticity. The blue-emitting phosphor emits blue light having a main emission peak wavelength of about 400 nm to about 500 nm. The green-emitting phosphor emits green light having a main emission peak wave length of about 500 nm to about 600 nm. The red-emitting phosphor emits red light having a main emission peak wavelength of about 600 nm to about 650 nm.
A blue-emitting phosphor BaMgAl10O17:Eu2+, a green-emitting phosphor LaPO4:Tb3+,Ce3+, and a red-emitting phosphor Y2O3:Eu3+are generally used as the blue, green-emitting, and red-emitting phosphors. Such phosphor materials are generally indicated by XX:YY, wherein XX represents the composition of a host material, and YY represents a luminescence center, in which part of atoms constituting the host material is replaced with the luminescence center. For example, the green-emitting phosphor LaPO4:Tb3+,Ce3+ includes LaPO4 as a host material, terbium (Tb) as a luminescence center, and cerium (Ce) as an intensifier for intensifying the light emission of terbium (Tb). In this green-emitting phosphor, terbium replaces part of lanthanum (La). Thus, LaPO4:Tb3+,Ce3+ may also be indicated as (La,Tb,Ce)PO4.
With reference to FIG. 8, visible rays radiated from the cold cathode fluorescent lamp (light source 5) passes through optical members arranged directly above the cold cathode fluorescent lamp, and come into the liquid crystal display panel 2 facing the back light unit 1. The optical members herein include the diffuser plate 6, the prism sheet 7, and a reflective polarizer 8. The reflector 4 is arranged directly below the cold cathode fluorescent lamp. The reflector 4 is configured to reflect light, and the reflected light also passes through the optical members and comes into the liquid crystal display panel 2.
The liquid crystal display panel 2 has a sectional structure as shown in FIG. 13. Specifically, the liquid crystal display panel 2 includes a pair of glass substrates 21 (21A and 21B), polarizers 22 (22A and 22B), alignment layers 23, a liquid crystal 24, color filters 25, and spacers 26. The pair of glass substrates 21A and 21B face each other. The alignment layers 23 are arranged on inner surfaces of the pair of substrates 21A and 21B, respectively. The color filters 25 include a red filter 25A, a green filter 25B, and a blue filter 25C. The liquid crystal 24 and the color filters 25A, 25B, and 25C are arranged between the pair of substrates 21A and 21B.
A gap between the pair of glass substrates 21 (21A and 21B) is held by the spacers 26. The polarizers 22 (22A and 22B) are arranged outside the pair of substrates 21 (21A and 21B), respectively. The liquid crystal 24 has a uniform alignment by the action of the alignment layers 23 and is driven upon application of a voltage to a group of electrodes (not shown in FIG. 13). The group of electrodes is arranged on a pixel basis. When a voltage is applied, an electric field is formed, the liquid rotates according to the electric field, so as to change the refractive index of the liquid crystal layer. Thus, the transmittance of light is controlled.
The color filters 25 (25A, 25B, and 25C) split white light W from the back light unit 1 into red light R, green light G, and blue light B and transmit any of these light on a pixel basis. Display modes of liquid crystals are classified by the initial alignment of liquid crystal molecules and the drive mode of liquid crystal molecules.
Representative display modes include an in-plane switching (IPS) mode, a vertically aligned (VA) mode, an optically compensated bend (OCB) mode, and a twisted nematic (TN) mode.
The IPS mode and the VA mode are widely employed in current large-screen television sets. According to the IPS mode, liquid crystal molecules are aligned substantially in parallel with the substrate plane and are driven by the action of an electric field substantially in parallel with the substrate plane. In contrast, according to the VA mode, liquid crystal molecules are aligned substantially vertical to a substrate plane and are driven by the action of an electric field substantially vertical to a substrate plane. The two modes can display images with excellent view angle characteristics and are suitable for use in large-screen liquid crystal display devices.
As is described above, liquid crystal display devices produce color displays by allowing a liquid crystal display panel to control, on a pixel basis, the transmittance of light from a light source in a back light unit and allowing color filters to split the light on a pixel basis, which color filters transmit any of red, green, and blue light.
Description on such green-emitting phosphor LaPO4:Tb3+,Ce3+can be found, for example, in the following documents.
Japanese Unexamined Patent Application Publication (Laid-Open) No. 57-23674;
Japanese Unexamined Patent Application Publication (Laid-Open) No. 4-338105;
Japanese Unexamined Patent Application Publication (Laid-Open) No. 5-302082;
Japanese Unexamined Patent Application Publication (Laid-Open) No. 6-56412;
Japanese Unexamined Patent Application Publication (Laid-Open) No. 9-249879;
Japanese Unexamined Patent Application Publication (Laid-Open) No. 2000-109826;
Japanese Unexamined Patent Application Publication (Laid-Open) No. 2002-212553; and
The Journal of Chemical Physics, Vol. 60, No. 1, p. 34(1974)